1. Field of the Invention
The present invention relates to a lead-free solder which is classified as a high-temperature solder and in particular to a lead-free, Sn-based solder suitable for use in soldering and precoating coil ends.
2. Description of the Related Art
Various coils are used in electronic equipment such as computers. For example, coils are incorporated into transformers or into motors for disc drives or cooling fans of electronic equipment. These coils are usually soldered at their ends to electric terminals to form electrical connections.
Normally coils are formed from copper wire covered with an insulating coating which typically consists of an inner enamel coat and an outer polyurethane resin coat. Therefore, in order to solder a coil at its ends to electrical terminals, it is necessary to remove both the inner enamel coat and the outer polyurethane coat (the two coats being hereinafter referred to collectively as insulating coats) from the copper wire at the coil ends. It is conceivable to remove the insulating coats at the coil ends by a mechanical method using a tool such as a knife, but such mechanical removal is slow, thus adversely affecting the operating efficiency of soldering. Therefore, removal of the insulating coats at coil ends is usually performed by a thermal method in which the insulating coats are melted away. More specifically, the insulating coats at coil ends are thermally removed by dipping the coil ends in molten solder at a temperature sufficient to melt the coats away (in the vicinity of 400° C.).
Soldering of coil ends is preferably preceded by precoating of the coil ends with solder in order to ensure that satisfactory soldered joints are formed. Such precoating is carried out by dipping the coil ends in molten solder.
Thus, the act of dipping the ends of a coil of coated copper wire in molten solder prior to soldering can perform two functions at the same time, i.e., removing the insulating coats from the wire at the coil ends and precoating the wire at the coil ends with solder.
Dipping of coil ends in molten solder is usually conducted after a soldering flux has been applied to the coil ends. The heat of the molten solder causes the insulating coats to melt into the molten solder, and the flux applied to the surface of the coil ends floats on the molten solder around the coil ends. As a result, as the insulating coats of the coil ends are removed by the heat of the molten solder to expose the copper wire, the flux floating around the coil ends acts on the exposed copper wire so as to assist the molten solder in mechanically bonding to the copper wire.
The coil ends which have been precoated with solder in this manner are then soldered to electric terminals. In some cases, soldering can be performed using the precoated solder alone. When the precoated solder is not sufficient to form a satisfactory soldered joint, additional solder is used to solder the precoated coil ends to electric terminals. The additional solder may be the same as that used for precoating, or it may be a different lead-free solder, which is preferably solder with a lower melting point.
Some copper wires used to make coils in electronic equipment are as thin as 100 μm or smaller in diameter. When the ends of such thin copper wires are dipped in molten solder, so-called copper leaching may take place. Copper leaching is the phenomenon in which a thin copper wire which is dipped in molten solder dissolves in the molten solder, thereby causing the wire to become appreciably thinner or completely disappear. Copper leaching may also occur during soldering of coil ends, particularly during dip soldering. Countermeasures against such copper leaching of thin copper wires have been taken in the past.
The technique that has been used most widely in order to prevent copper leaching of a thin copper wire in molten solder is to add Cu to the solder. Since the amount of Cu which can dissolve in molten solder is limited, use of a solder to which Cu has been added makes it difficult for a copper wire to dissolve in molten solder when it is dipped in the molten solder. An example of such a solder is a Pb—Sn—Cu alloy, and a typical composition of such solder is a Pb-63Sn alloy to which Cu is added so as to have a Cu content of about 1.5% to 2%. The Pb—Sn—Cu solder has good solderability with respect to copper wires, so it has been widely used in precoating of coil ends.
However, because of the toxic nature of lead (Pb), which may result in lead poisoning when lead-containing waste is disposed of in a landfill and acid rain dissolves lead from the waste and contaminates underground water with lead, there is a need for a lead-free solder for use in precoating and soldering of coil ends.
Lead-free solders which have been proposed in the past as being capable of preventing copper leaching are Sn-based alloys containing Cu, which is an element known to be effective for prevention of copper leaching. For example, an Sn-based lead-free solder comprising, by mass %, 5.5–8.0% of Cu, optionally 0.01–1.00% of Ag and 0.001–0.010% of Ni, and a remainder of Sn is described in JP P2001-121286A. This solder may further contain a minor amount of one or both of Ge and Au. Another Cu-containing, Sn-based lead-free solder is described in JP P2001-334384A. That solder comprises, by mass %, 0.01–0.5% of Ni, 2–5% of Cu, and optionally one or more of 0.01–3.5% of Ag, 0.01–5% of Sb, 0.01–9% of Zn, 0.01–3% of Bi, 0.01–0.5% of Ge, and 0.01–0.5% of P, and a remainder of Sn.
The element that is responsible for copper leaching is Sn, since copper leaching is caused by dissolving Cu in Sn. Therefore, with the above-described prior-art Sn-based lead-free solders, copper leaching generally occurs more markedly due to its higher Sn content compared to with a conventional Cu-containing Pb—Sn solder. Furthermore, since such Sn-based lead-free solders have a higher liquidus temperature so that the soldering temperature or the temperature of molten solder is also higher, copper leaching becomes still more severe. As a result, the effect of these prior-art Sn-based lead-free solders is not sufficient to achieve prevention of copper leaching at coil ends to a satisfactory degree, particularly when the coil is formed from a thin copper wire with a diameter of at most 100 μm.